The technical field of this invention is corneal surgery and, in particular, the invention relates to systems and methods for reprofiling the Bowman's membrane of the cornea of the eye.
The cornea of the eye is composed of a set of distinct layers: the outer epithelium, an anterior elastic lamina known as "Bowman's layer", the cornea proper (or "stroma"), a posterior elastic lamina known as "Descemet's membrane", and the inner endothelium. The stroma is fibrous and transparent and constitutes the major portion of the cornea. Bowman's layer, which forms the outer elastic lamina, is a rigid fibrillar structure not tending to cut or fracture, while Descemet's membrane, which forms the inner elastic lamina, is very brittle but elastic and has a tendency to curl. Together, the Bowman'layer and Descemet's membranes impart the necessary curvature to the stromal tissue.
In the field of surgery, a known technique for treatment of certain forms of refractive errors, such as acute myopia, hyperopia and astigmatism, is to surgically remove an anterior segment of the cornea down into the stroma, to reshape the removed segment as by surgical grinding in a frozen state, and to restore the reshaped segment into the eye. In this type of operation, known as keratoplasty, the eye heals by reformation of the outer epithelium layer over the reshaped stroma. Alternatively, a layer of the cornea can be opened up as a flap, an artificial or donor lenticular implant then inserted under the flap, and the flap sutured up again.
Other surgical techniques for altering the corneal surface to correct refractive errors have also been proposed. One increasingly common technique is Radial Keratectomy ("RK") in which a set of radial incisions, i.e., resembling the spokes of a wheel, are made in the eye, down into the stroma, to remedy refractive errors such as myopia (nearsightedness). As the incisions heal, the curvature of the eye is flattened, thereby increasing the ocular focal distance. The operation is not particularly suitable for correction of hyperopia (farsightedness) and can pose problems if the surgical incisions are uneven or too deep.
Until recently, surgical operations on the cornea were most commonly carried out using diamond or steel knives or razors, and such techniques continue to be practiced. For example, use of a physical cutting tool is still widespread in corneal operations such as keratoplasty and lenticular implants (See, generally, Binder et al, "A Refractive Keratoplasty," 100 Arch. Ophthalmol., 802 (1982) and "Refractive Keratoplasty Improves With Polysulfone Pocket Incision," Ophthalmology Times (July 1, 1986)).
Use of the laser beam as a surgical tool for cutting non-ocular incisions, a so-called "laser scalpel", has been known for some time. (See, for example U.S. Pat. No. 3,769,963 to Goldman et al). However, the utility of most forms of laser radiation for corneal surgery is compromised by the tendency of laser beams to cause thermal damage and, consequently, scaring and opacification in the extremely delicate structure of the corneal stroma.
Even "cold" photoablative UV radiation from excimer lasers and the like must be carefully controlled to avoid permanent damage to the eye. (For a study of damage which can be inflicted on the cornea by exposure to uncontrolled excimer laser radiation, see Taboada et al, "Response of the Corneal Epithelium to ArF Excimer Laser Pulses," 40 Health Physics 677-83 (1981)). Nonetheless, the use of excimer laser radiation to replace conventional physical cutting tools in many corneal surgical procedures holds significant promise.
A new technique for corneal reshaping involves the use of an excimer laser photoablation apparatus in which the size of the area on the surface to which the pulses of laser energy are applied, is varied to control the reprofiling operation. In one embodiment, a beam-shaping stop or window is moved axially along the beam to increase or decrease the region of cornea on which the laser radiation is incident. By progressively varying the size of the exposed region, a desired photoablation profile is established on the surface. For further details on this technique, see Marshall et al, "Photo-Ablative Reprofiling of the Cornea Using an Excimer Laser: Photorefractive Keratectomy," 1 Lasers in Ophthalmology, 21-48 (1986), and the above-referenced, commonly-owned, copending U.S. patent application Ser. Nos. 869,335 and 905,156, herein incorporated by reference.
Another new technique for corneal reshaping involves the use of a laser photoablation apparatus in which a beam-shaping mask is disposed between the laser and the surface. In one embodiment, the mask provides a predefined profile of resistance to erosion by laser radiation whereby a portion of the laser radiation is selectively absorbed and another portion is transmitted to the surface in accordance with the mask profile. For further disclosure of such erodible masking techniques, see the above-referenced, commonly-owned, copending U.S. patent application Ser. Nos. 019,200 and 124,101, also incorporated herein by reference.
It also has been suggested that controlled ablative photo-decomposition of one or more selected regions of a cornea can be performed using a scanning action on the cornea with a beam from an excimer laser (see, for example, U.S. Pat. No. 4,665,913 issued to L'Esperance on May 19, 1987). In the L'Esperance patent, it is suggested that myopic and hyperopic conditions can be reduced by repeatedly scanning the cornea with an excimer laser beam to achieve penetration well into stroma and to induce resculpting of the stroma tissue.
However, the approach of penetrating into the stroma of the cornea in order to correct refractive errors is considered by many clinicians to be an extremely risky procedure. Typically, the stroma of the cornea is only about 500 microns in thickness and a mishap in the photoablative process could result in destruction of the underlying Descemet's membrane and/or the endothelial lining with consequent permanent loss or impairment of sight.
Therefore, it is an object of the present invention to provide an improvement whereby laser techniques can be applied to the eye where it is desirable to effect corrections of refractive errors while minimizing the risks inherent in surgical ablation of stroma tissue. It is a further object of this invention to provide an improved and less traumatic method of reshaping the cornea of the eye.